The present invention relates to a physical information acquisition method, a physical information acquisition apparatus, and a semiconductor device. More particularly, the present invention relates to a driving control technique in reading unit-element signals from unit elements, particularly suitable for use in a semiconductor device, such as a solid-state image sensor including an array of unit elements sensitive to an electromagnetic wave such as light or radiation incident from the outside and capable of outputting an electrical signal indicating a physical quantity distribution detected by the unit elements.
In various applications, to detect a physical quantity distribution, a semiconductor device is widely used which includes a linear array or a matrix array of unit elements (pixels) sensitive to a change in a physical quantity such as a pressure or an electromagnetic wave such as light or radiation incident from the outside.
For example, in video devices, a solid-state image sensor is used which includes an image sensor device of a CCD (Charge Coupled Device) type, a MOS (Metal Oxide Semiconductor) type, or a CMOS (Complementary Metal-Oxide Semiconductor) type to detect a change in a physical quantity such as light (which is an example of an electromagnetic wave).
In computer devices, a fingerprint recognition device is used to acquire fingerprint information by detecting an image of a fingerprint based on a change in an electrical or optical characteristic associated with a pressure. In these apparatus, a physical quantity distribution is converted into an electrical signal by unit elements (pixels in the case of a solid-state image sensor) and the resultant electrical signal is read out.
In some solid-state image sensors, an active pixel sensor is used in which a driving transistor for amplification is disposed in each image signal generation part that generates an image signal corresponding to a signal charge generated in a charge generation part. This structure is used in many CMOS solid-state image sensors.
In such an active solid-state image sensing apparatus, to read an image signal, unit pixels arranged in a pixel array part are sequentially selected by controlling addressing, and signals are read from the respective unit pixels. That is, the active solid-state image sensing apparatus is a solid-state image sensor of the address control type.
For example, in an active pixel sensor of the X-Y address type in which unit pixels are arranged in the form of a matrix array, each pixel is configured to have an amplification capability using an active element having a MOS structure (MOS transistor). In this structure, a signal charge (photoelectrons) accumulated in a photodiode serving as a photoelectric conversion device is amplified by the active element and read out as image information.
In the X-Y addressing solid-state image sensing device of this type, for example, a pixel array part is formed using a large number of pixel transistors arranged in the form of a two-dimensional matrix array. Accumulation of signal charges corresponding to incident light is started on a line-by-line (row-by-row) basis or a pixel-by-pixel basis, and a current or a voltage corresponding to the signal charge accumulated in each pixel is read sequentially from the respective pixels by accessing the pixels by means of addressing. In solid-state image sensing devices of the MOS type (and of the CMOS type), the addressing is performed, for example, such that pixels are simultaneously accessed on a line-by-line basis and pixel signals are read from the accessed pixels, that is, pixel signals are read on a line-by-line basis from a pixel array part.
In some solid-state image sensing devices of this type, to adapt to the reading scheme of accessing the pixel array part on a line-by-line basis and reading pixels signals from the accessed line, analog-to-digital converters and/or other signal processing units are disposed for respective vertical columns. This configuration is called a column parallel arrangement. Of solid-state image sensing devices with a column parallel arrangement, a solid-state image sensing device in which a CDS processor or a digital converter is disposed in each vertical column such that pixel signals are sequentially read and output is called a column-type solid-state image sensing device.
As a result of reductions in size and cost of solid-state image sensing devices such as CCD or CMOS image sensors, various kinds of video devices using a solid-state image sensing device, such as a digital still camera for taking a still image, a portable telephone with a camera, and a video camera for taking a motion image, have come to be widely used. CMOS image sensors can operate with less consumption power and can be produced at a lower cost than CCD image sensors, and thus CMOS image sensors are expected to be widely used instead of CCD image sensors.
In recent years, a great advance in semiconductor technology has been made, and, as a result, an increase in the number of pixels of solid-state image sensing devices has been achieved. For example, solid-state image sensing devices having several hundred pixels are now available and used in high-resolution digital still cameras and movie video cameras.
The increase in resolution results in an increase in the number of pixel transistors. The increase in the number of pixel transistors and an increase in the number of functions achieved by the capability of accessing arbitrary pixels result in an increase in the length of control lines for controlling reading of pixel signals. This causes an increase in load imposed on drivers connected to the control lines and also causes an increase in skew, which cannot be neglected.
For example, in CMOS image sensors, electrons generated as a result of photoelectric conversion are accumulated in each pixel, and pixel signals are sequentially read from pixels in pixel columns (vertical columns) specified by address control signals output from a sensor control unit (SCU).
More specifically, an address decoder is disposed in a vertical scanning circuit located close to the pixel array part, and an address control signal is supplied from the address decoder to sequentially select pixels. In accordance with the address control signal, the vertical scanning circuit supplies various kinds of control signals (generically it can be referred to as control signals) to a predetermined points on drive control lines (particularly they can be referred to as original driving points) via driving buffers. And then, the control signals go to pixel transistors, which are connected to respective driving points on the drive control lines, through the drive control lines, thereby turning on/off the pixel transistor at the specified horizontal address position. Thus, the address decoder generates data indicating the address of a pixel to be selected.
Various control signals, by which to specify the horizontal address position, control turning on/off of the pixel transistor, are transmitted via control signal lines, and pixel signals output from pixels in units of lines are sequentially transmitted in a horizontal direction via a horizontal signal line (horizontal transfer line). When there are a large number of pixels, these control signal lines and horizontal signal line extend a long distance across the whole pixel array part, and thus an interval between the original driving point and the respective driving points where each pixel is connected get longer. Accordingly skew caused by the difference in locations of pixels along these control signal lines or the horizontal signal line becomes very serious.
The skew can cause a reduction in a timing margin in an operation of shading in a horizontal direction or in an operation of transferring data to an amplifier at a following stage. Therefore, it is desirable to minimize the skew to as low a level as possible.
For example, a tree layout such as that shown in FIG. 10 is used to equally distribute a drive control signal (clock signal) in a sensor. In this layout, the overall skew of the circuit is dominated by a skew that occurs at a first stage having a longest interconnection. Thus, it is desirable to minimize the skew at the first stage.
A widely used technique of driving the same line using one or two driving buffers (pixel drivers) is to dispose one or two driving buffers at one or both ends of the line and drive the pixels using the driving buffers.
When pixels are driven from one side with one driving buffer (an example of driver unit) being connected to one end of the drive control lines, the distance between the driving buffer to the pixel varies greatly depending on the location on the line. Thus, a difference in arrival time of a driving pulse (skew) occurs among pixels depending on the locations of the pixels. That is, a difference in arrival time of the driving pulse occurs between pixels located close to the driving buffer and pixels located far from the driving buffer. This can make it impossible to read pixel signals or can cause shading.
When pixels are driven from both sides with two driving buffers (an example of driver unit) being connected to both ends of the drive control lines, the dependence of the distance from the driving buffer to the pixel on the location on the line becomes smaller than in the case in which pixels are driven from one side. However, even when pixels are driven from both sides, the dependence of the distance can be still large. That is, when pixels are driven from both sides, it becomes more difficult to read a signal from a pixel located at the center of the line as the number of pixels increases and/or as the signal reading rate increases. This is a serious problem to be solved when the signal reading rate is increased.